The invention relates to an induction circuit for a contactless power supply apparatus.
Please refer to FIG. 1, which illustrates a related art induction circuit for a contactless power supply apparatus as disclosed in JP. Pat. No. 11-252912. In this related art induction circuit, a secondary induction circuit comprises an induction coil 92 for inducing a current in response to a high frequency current flowing through a primary induction circuit 91. The induction coil 92 is coupled to a resonant capacitor 93 to form a resonant circuit wherein the resonant circuit has a resonant frequency the same as the frequency of the high frequency current in the primary induction circuit 91. The resonant capacitor 93 is connected to a full-wave rectifier 94 wherein the full-wave rectifier 94 is coupled to a constant-voltage control circuit 95 which controls an output voltage according to a reference voltage. A load 96 is coupled to the constant-voltage control circuit 95 and supplied power thereby. The two diodes 94A and 94B in the full-wave rectifier are connected to the short-circuited transistors 921A and 921B respectively.
A current-limiting coil 97 is coupled between cathodes of the two diodes 94C and 94D, and an adjusting transistor 910 coupled to the outputs of the full-wave rectifier 94. The constant-voltage control circuit 95 further comprises a comparator 99 and a timer 922, wherein the comparator 99 compares the output and reference voltages Vref and generates a first output signal to timer 922 and an AND circuit 923 according to the comparison result. The timer 922 starts to count in response to the first output signal and generates a second output signal to the AND circuit 923. Thus the AND circuit 923 controls the adjusting transistor 910 in accordance with the first and second output signals.
The operation of the induction circuit is described in the following. When the load 96 is reduced and the output voltage VDC rises, the comparator 99 generates the first output signal to turn on the adjusting transistor 910. Meanwhile, the first output signal turns on the timer 922. Timer 922 then generates the second output signal for turning on the short-circuited transistors 921A and 921B and turning off the adjusting transistor 910 after a predetermined time. When the adjusting transistor 910 is turned on, the output voltage VDC is held at the reference voltage level vref. Furthermore, the energy accumulated in the resonant capacitor is accumulated in the current-limiting coil 97. When the short-circuited transistors 921A and 921B are turned on and the adjusting transistors 910 are turned off, the energy in the current-limiting coil 97 is output to the load 96 to be used effectively, thus reducing power loss.
The excessive output voltage VDC, however, may be reduced to the reference voltage level vref by the constant-voltage control circuit 95 when the adjusting transistor 910 in the constant-voltage control circuit 95 is turned on and the charges accumulated in the resonant capacitor flow into the adjusting transistor 910 via the current-limiting coil 97. This impulse current may damage the adjusting transistor 910 and heat the current-limiting coil 910 at a high speed, causing heat loss. The sizes of the current-limiting coil 97 and the adjusting transistor 910 are also limited due to the mentioned impulse current. In addition, there is a problem of complexity of the short-circuited transistor 921A and 921B, and the adjusting transistor 910 in the related art.
In view of the above, the objective of the invention is to provide an induction circuit for a contactless power supply apparatus to prevent transistors from damage when an impulse current is generated, reduce the heat loss of coils and minimize the size of components.